The present invention relates generally to a means for increasing the cycle performance of a vapor compression system by using the work produced by the expansion of high or intermediate pressure refrigerant to drive an expander motor coupled to auxiliary rotating machinery.
Chlorine containing refrigerants have been phased out in most of the world due to their ozone destroying potential. Hydrofluoro carbons (HFCs) have been used as replacement refrigerants, but these refrigerants still have high global warming potential. xe2x80x9cNaturalxe2x80x9d refrigerants, such as carbon dioxide and propane, have been proposed as replacement fluids. Unfortunately, there are problems with the use of many of these fluids as well. Carbon dioxide has a low critical point, which causes most air conditioning systems utilizing carbon dioxide to run transcritical under most conditions.
When a typical vapor compression system runs transcritical, the high side pressure of the refrigerant is high enough that the refrigerant does not change phases from vapor to liquid while passing through the heat rejecting heat exchanger. Therefore, the heat rejecting heat exchanger operates as a gas cooler in a transcritical cycle rather than as a condenser. The pressure of a subcritical fluid is a function of temperature under saturated conditions (where both liquid and vapor are present).
In a transcritical vapor compression system, refrigerant is compressed to a high pressure in the compressor. As the refrigerant enters the gas cooler, heat is removed from the high pressure refrigerant. Next, after passing through an expansion device, the refrigerant is expanded to a low pressure. The refrigerant then passes through an evaporator and accepts heat, fully vaporizes, and re-enters the compressor completing the cycle.
In refrigeration systems, the expansion device is typically an orifice. It is possible to use an expander unit to extract the energy from the high pressure fluid. In this case, the expansion of the refrigerant flowing from the gas cooler or condenser and into the evaporator converts the potential energy in the high pressure refrigerant to kinetic energy, producing work. If the energy is not used to drive another component in the system, it is lost. In prior systems, the energy converted by the expansion of the refrigerant drives an expander motor unit coupled to the compressor to either fully or partially power the compressor. The expansion of pressurized cryogen has also been used in prior systems to drive mechanical devices in refrigerant units, but not in vapor compression systems.
A reversible vapor compression system includes a compressor, a first heat exchanger, an expansion device, an expansion motor unit coupled to auxiliary rotating machinery, a second heat exchanger, and a device to reverse the direction of refrigerant flow. By reversing the flow of the refrigerant with the heat pump, the vapor compression system can alternate between a heating mode and a cooling mode. Preferably, carbon dioxide is used as the refrigerant. Because carbon dioxide has a low critical point, systems utilizing carbon dioxide as a refrigerant usually require the vapor compression system to run transcritical.
The high pressure or intermediate pressure refrigerant exiting the gas cooler is high in potential energy. The expansion of the high pressure refrigerant in the expansion device converts the potential energy into useable kinetic energy which is utilized to completely or partially drive an expansion motor unit. The expansion motor unit is coupled to drive auxiliary machinery. By employing the kinetic energy converted by the expansion of the high pressure or intermediate pressure refrigerant to fully or partially drive the expansion motor unit coupled to the auxiliary machinery, system efficiency is improved. The auxiliary machinery can be an evaporator fan or a gas cooler fan which draw the air through the evaporator and gas cooler, respectively. Alternatively, the auxiliary machinery can be a water pump which pumps the water or other fluid through the evaporator or gas cooler that exchanges heat with the refrigerant. The auxiliary machinery can also be an oil pump used to lubricate the compressor.